1. Field of the Invention
The present invention relates to a key switch disposed on a keyboard input device, and a method of producing the same.
2. Description of the Related Art
In recent years, various types of key switches suitable for reducing the thickness of a keyboard input device have been proposed. The various types of key switches are constructed so that a key top is supported at the top ends of a pair of lever members linked together in such a way as to cross each other, and the crossing angle of the pair of lever members changes with the raising and lowering of the key top.
For example, there is disclosed a key switch having a structure which uses a cross-link member for guiding the raising and lowering operation of a key top. In the cross link member, lever members are linked together at intersection portions thereof. The top end of one of the lever members rotatably engages the bottom surface of the key top, whereas the top end of the other lever member slidably engages the bottom surface of the key top.
In this type of key switch, when an operator depresses a key top, the pair of lever members are lowered, causing the cross-link member to be pushed downward. When the key top has descended by a predetermined amount, a resilient member, such as a click rubber, is pushed by the key top and gets buckled, so that a switch element, such as a membrane switch, is pushed by the resilient member, causing the switch to be turned on. When the pressing force on the key top of the switch in an ON state is removed, the buckled resilient member returns to its original shape as a result of its resiliency. Therefore, the switch element is restored to the OFF state, and the key top is pushed upward to its initial position as the lowered pair of lever members are raised.
Compared to a commonly used structure in which a key stem is slid along a guide wall, such a structure in which a key top is supported by a cross-link member so that it can be raised and lowered makes it possible to ensure ease of operation and to considerably reduce the height of the key switch.
In the conventional key switch that includes a cross-link member in which a pair of lever members are linked so that they intersect each other, the cross-link member is formed as a result of linking both lever members at their intersection portions after they have been molded. Here, it is necessary to carry out an assembly operation in which a linking shaft provided on one of the lever members is inserted into a shaft-receiving hole formed in the other lever member in order to pivotally mount it thereto. However, in recent years, key switches are becoming smaller in size, so that lever members of cross-link members are becoming considerably smaller in size, making the assembly operation troublesome to carry out because small molded lever members need to be manually linked together in order to form a cross-link member. In particular, when the production of smaller key switches is accelerated, it inevitably becomes extremely difficult to link a pair of lever members by a manual operation.
In view of the above-described conventional problems, it is an object of the present invention to provide a key switch which makes it possible to easily and reliably link a pair of lever members which support a key top in order to form a cross-link member without having to link them by a manual operation, and which makes it possible to accelerate size reduction and to perform an assembly operation with greater ease. It is also an object of the present invention to provide a method of producing the same.
To these ends, according to one aspect of the present invention, there is provided a key switch comprising a pair of lever members which are rotatably linked together at crossing portions thereof by pivotally mounting a linking shaft in a shaft-receiving hole, a key top which supports the pair of lever members so as to raise and lower the lever members, a resilient member for biasing the key top upwards, and a switch element for performing a switching operation with the raising and lowering of the key top. In the key switch, the pair of lever members are molded out of resin materials having different shrinkage coefficients. The shaft-receiving hole is formed in the lever member formed of the resin material having a small shrinkage coefficient. The linking shaft is formed at the lever member formed of the resin material having a large shrinkage coefficient.
In the key switch having this structure, a conventionally known two-color molding technology is used to successively mold the pair of lever members in order to form the cross-link member. Therefore, it is no longer necessary to manually link the pair of lever members. Consequently, even if the lever members are small due to the size reduction of the key switch, a troublesome assembly operation does not need to be carried out.
More specifically, in a method of producing such a key switch in accordance with the present invention, the lever member which has the shaft-receiving hole formed therein is molded as a result of hardening a first resin material in a die. Then, a second resin material having a shrinkage coefficient which is greater than that of the first resin material is hardened in a die in which the shaft-receiving hole forms a portion of a cavity in order to mold the other lever member. By molding the other lever member, the linking shaft is formed in a pivotally mounted state in the shaft-receiving hole.
Accordingly, when the lever member which is formed of a resin material having a small shrinkage coefficient and which has the shaft-receiving hole is molded, and then a resin material having a large shrinkage coefficient is used for molding where the shaft-receiving hole forms a portion of a cavity, the linking shaft is formed in a pivotally mounted state in the shaft-receiving hole at the stage of cooling and hardening of the resin material. Therefore, the pair of lever members can be easily and reliably linked together without carrying out a manual operation in order to form the cross-link member.